Measurement flow section for oil well effluents and system including such a section

ABSTRACT

The invention relates to a flow section for measurements concerning oil well effluent, the section comprising a pipe in parallel with a pipe connected to the well head. The flow section includes elements of material having low attenuation for gamma rays fixed in recesses formed in the wall of the pipe on its inside, and protection members disposed in housings connecting said recesses respectively to the outside, said housings being suitable for receiving gamma ray attenuation measuring means after the protection members have been removed.

BACKGROUND OF THE INVENTION

The invention relates in general to measurements for determining at least one characteristic of oil well effluent constituted by multiphase fluid which typically comprises three phases: two liquid phases, crude oil and water, plus a hydrocarbon gas phase. In particular, the characteristics in question are the proportions of the component phases, including the water content of the liquid phase, and the flow-rate values—the total flow rate and the flow rates of the various phases.

In the oil industry, the traditional practice is to separate the effluent into its component phases and to perform measurements on the phases separated in this way. However that technique requires separators to be installed on site, which separators are bulky and expensive items of equipment, and when testing wells, it also requires additional pipes to be put into place.

Numerous proposals have been put forward for developing techniques that would make it possible to avoid using such separators. A description of these developments is to be found in SPE publication 28515 (SPE Annual Technical Conference, New Orleans, Sep. 25-28, 1994) by J. Williams, “Status of multiphase flow measurement research”.

Amongst such proposals, U.S. Pat. No. 4,788,852 describes apparatus including a device for measuring gamma ray attenuation, that device being associated with a Venturi total flow rate sensor and being situated at the constriction of the Venturi. Apparatus of that type is also described in patent application WO 94/25859 and in SPE publication 36593 dated Oct. 6, 1996, “Multiphase flow measurement using multiple energy gamma ray absorption (MEGRA) composition measurement” by A. M. Scheers and W. F. J. Slijkerman.

SUMMARY OF THE INVENTION

The invention seeks to provide gamma ray attenuation measurements, either alone or combined with total flow rate measurements, in advantageous conditions from an operational standpoint.

In one aspect, the invention provides a flow section adapted for measurements of oil well effluent, the section comprising a pipe in parallel with a pipe connected to the well head, elements of material having low gamma ray attenuation fixed in recesses formed on the internal wall of the pipe, and protection members disposed in housings connecting said recesses respectively to the outside, said housings being suitable for receiving gamma ray attenuation measuring means after the protection members have been removed.

In appropriate manner, sealing means are provided in association with said elements.

The invention will be well understood on reading the following description given with reference to the accompanying drawings. The list of drawings is as follows:

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagrammatic view of a measurement device for characterizing an oil well effluent, including a Venturi section and a device for measuring gamma ray attenuation;

FIG. 2 is a longitudinal section through an embodiment of the FIG. 1 device;

FIG. 3 is a detail view showing a portion of the device shown in FIG. 2, but on a larger scale; and

FIG. 4 is a diagram of a Venturi section as shown in FIGS. 2 and 3, and fitted for semi-periodic use.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 1 shows in highly diagrammatic manner a flow meter device adapted to oil well effluent.

The device comprises a pipe section 10 comprising a converging Venturi 11 whose narrowest portion 12 is referred to as the throat. In the example shown, the section of the pipe 10 is disposed vertically and the effluent flows upwards, as symbolized by arrow F.

The constriction of the flow section in the Venturi induces a pressure drop Δp between the level 13 situated upstream from the Venturi at the inlet to the measurement section and the throat 12. This pressure drop is associated with the total mass flow rate Q and with the density ρ_(m) by the following relationship: $\begin{matrix} {{\Delta \quad p} = {\frac{K \cdot Q^{2}}{\rho_{m}} + {\rho_{m}{gh}_{v}}}} & (1) \end{matrix}$

where g is the acceleration due to gravity, h_(V) is the distance between the upstream level 13 and the throat 12, and K is a constant associated essentially with the geometry of the Venturi, and which is given by: $K = \frac{1 - \beta^{4}}{2C^{2}A^{2}}$

where β is the constriction ratio of the Venturi, i.e. the ratio between the diameter of the throat and the upstream diameter of the Venturi, C is the discharge coefficient, and A is the section of the throat. The term ρ_(m).g.h_(V) is generally small or negligible. By writing Δp*=Δp−ρ_(m).g.h_(V), relationship (1) becomes:

Q=k(Δp*.ρ _(m))^(½)  (2)

where k=K^(−½)

The density ρ_(m) is measured at the throat of the Venturi. This is relevant to the validity of relationship (2) for the following reason. The acceleration, and consequently the pressure drop, to which the fluid in the Venturi is subject takes place in privileged manner in the region close to the throat, because velocity, which is proportional to the square of the diameter, increases considerably in this region. Relationship (2) normally assumes a single phase fluid. It remains suitably applicable to a multiphase fluid providing density ρ_(m) is measured at the throat. This is particularly true with increasing Venturi effect, and for this reason, an appropriate value for the constriction ratio is β=0.5. With a pipe having a diameter of 10 cm, the diameter in the throat is then 5 cm.

The discharge coefficient C is approximately 1. It depends to a small extent and in predictable manner on the properties of the fluid. Traditionally, this corrective effect is taken into account by the Reynolds number.

The pressure drop Δp is measured by means of a differential pressure sensor 15 connected to two pressure takeoffs 16 and 17 opening out into the measurement section respectively at the upstream level 13 and in the throat 12 of the Venturi. In a variant, the measurement may also be performed by means of two absolute pressure sensors connected to the pressure takeoffs 16 and 17, respectively.

The density ρ_(m) of the fluid mixture is determined by means of a sensor which measures the attenuation of gamma rays, by using a source 20 and a detector 21 placed on opposite sides of the Venturi throat 12. The throat is provided with “windows” of material of low photon absorption at the energies under consideration. The source 20 produces gamma rays at two different energy levels, referred to below as the “high energy” level and as the “low energy” level. The detector 21 which comprises in conventional manner a scintillator crystal such as NaI and a photomultiplier produces two series of signals W_(hi) and W_(lo) referred to as count rates, representative of the numbers of photons detected per sampling period in the energy ranges bracketing the above-mentioned levels respectively.

These energy levels are such that the high energy count rate W_(hi) is essentially sensitive to the density ρ_(m) of the fluid mixture, while the low energy count rate W_(lo) is also sensitive to the composition thereof, thus making it possible to determine the water content of the liquid phase.

It is suitable for the high energy level to lie in a range 85 keV to 150 keV. For characterizing oil effluent, this energy range presents the remarkable property whereby the mass attenuation coefficient of gamma rays therein is substantially the same for water, for sodium chloride, and for oil, being about 0.17 cm²/g. This means that the high energy attenuation makes it possible to determine the density ρ_(m) of the fluid mixture without that requiring auxiliary measurements to be performed to determine the properties of the individual phases of the fluid mixture (attenuation coefficients and densities).

The attenuation measured by the detector 21 is expressed by the following relationship:

A=D _(v).ν_(m).ρ_(m)   (3)

where D_(V) is the distance travelled through the fluid, i.e. in this case the diameter of the Venturi throat, and ν_(m) is the mass attenuation coefficient of the fluid mixture.

Since the mass attenuation coefficients of water and oil in the energy range above are substantially identical, and since the contribution of the gas is negligible because of its very low density, the mass attenuation coefficient ν_(m), and thus the product D_(V).ν_(m) that appears in equation (3) can be considered as being substantially constant and independent of the densities ρ_(o) and ρ_(w) of the oil and water phases.

Under such conditions, the high energy attenuation A_(hi) is a very advantageous indicator of the density ρ_(m) of the mixture.

A material that is suitable for producing high energy gamma rays in the energy range under consideration, and low energy rays is gadolinium 153. This radioisotope has an emission line at an energy that is approximately 100 keV and that is entirely suitable for use as the high energy source. Gadolinium 153 also has an emission line at about 40 keV, which is suitable for the low energy level that is used to determine water content. This level provides good contrast between water and oil, since the attenuation coefficients at this level are significantly different, typical values being 0.228 cm²/g for oil and 0.291 cm²/g for sea water. It is also well separated from the high energy level and well above the noise level of the detector.

On the topic of the above-described gamma ray attenuation sensor, it should be observed that uses can be envisaged therefor other than those described above. The sensor may be used on its own and thus only provide water content information, in which case the sensor may be mounted in a straight section of pipe, or it may be combined with a flow rate sensor of a type other than a Venturi sensor. As an example of such sensors, mention can be made in particular of devices in which, as for a Venturi, a change of flow velocity is induced and the resulting pressure drop is measured (perforated plates).

FIG. 1 also shows a pressure sensor 22 connected to a pressure takeoff 23 opening out into the throat 12 of the Venturi, which sensor produces signals representative of the pressure ρ_(v) in the throat of the Venturi, and a temperature sensor 24 producing signals T representative of the temperature of the fluid mixture. The data ρ_(v) and T is used in particular for deriving gas density ρ_(g) under line conditions and gas flow rate q_(g) under normal conditions of pressure and temperature from the value for the flow rate under line conditions, determined in a manner described below. In this respect, it is preferable for the pressure to be measured at the throat of the Venturi. In contrast, it does not matter where temperature is measured.

Finally, a block 30 represents a unit for acquiring and processing data, which unit receives the signals coming from the above-mentioned sensors.

Description with Reference to FIGS. 2 and 3

FIG. 2 is a section through an embodiment of the Venturi section 10 shown diagrammatically in FIG. 1, and FIG. 3 shows in greater detail how the elements 20 and 21 of the gamma attenuation measuring device are assembled at the throat of the Venturi. Elements corresponding to elements of FIG. 1 are designated herein with the same reference numeral plus 100. The Venturi section 110 thus includes a converging portion 111 whose throat 112 is the narrowest portion thereof.

In FIG. 2, there can be seen pressure takeoff orifices 116 disposed at the upstream level 113 of the Venturi. Other pressure takeoff orifices 117 are to be found at the level of the throat 112. Each of these orifices is provided with a bore 125 opening out into the throat 112, and with a tapped hole 126 of larger diameter in which it is possible to fix the endpiece of a duct (not shown), in order to put the flow into communication, as appropriate, with a differential pressure sensor or with an absolute pressure sensor such as the sensors 15, 22 mentioned with reference to FIG. 1. The number of orifices is optional. FIG. 2 shows two pairs of orifices 116 disposed at 90°, however an appropriate measurement can be obtained with a single orifice 116.

FIG. 2 shows the gamma ray attenuation measurement device constituted by a source block 120 and a detector block 121, both assembled to the Venturi section. These elements have a longitudinal axis 118 disposed perpendicularly to the axis 114 of the Venturi section at its throat 112 and they are shown in longitudinal section in FIG. 3. FIG. 3 shows in greater detail firstly the housings 130 and 131 opening out into the outside wall of the Venturi and provided with threads to enable the source block 120 and the detector block 121 to be screwed respectively thereto, and also showing housing 132 and 133 opening out into the inside wall of the Venturi and receiving the “windows” (pieces of material presenting low gamma ray attenuation) 134 and 135, together with the associated elements. Each window is held in place by a tubular element 136 screwed into a larger diameter threaded portion 137 of the corresponding housing, together with a gasket 138 held against a shoulder in the housing. This arrangement of the windows provides a sealing barrier and enables the source block 120 and the detector block 121 to be implemented in the form of removable elements that can be dismounted in complete safety. In an appropriate embodiment, the windows 134 and 135 are made of beryllium, a material that presents low gamma ray attenuation at the energies under consideration, coated with a protective layer of boron hydride, a material which is highly resistant to corrosion and wear and which also has low gamma ray attenuation. It should be observed that the windows 134 and 135 which are described above as being distinct elements could also be implemented in the form of diametrically opposite distinct portions of a single annularly-shaped part housed in a recess of complementary shape formed inside the Venturi section. Under such circumstances, the gasket would naturally have the appropriate annular shape.

The source block 120 as shown in FIG. 2 is adapted to using a radioisotope having a short lifetime, such as the above-mentioned gadolinium 153 whose half-life is about 7 months, since it enables declining activity of the source to be compensated by reducing the distance of the source from the throat 112 of the Venturi section.

In the embodiment shown, the source block comprises a body 140 of tubular shape having a central bore that is closed by a plug 141 at its end remote from the Venturi section 110, and provided at its other end with a portion 14 of smaller diameter suitable for being screwed into the above-mentioned housing 130, said portion 142 having an O-ring 143 on its front face. The central bore is closed at its end at the portion 142 by a path 144 of material of low gamma ray attenuation, and that bears against a ring 145. The source proper 146, preferably a gadolinium 153 source having activity of 100 millicuries, for example, is centered in the central bore, and is displaceable along the longitudinal axis 118 by means of an adjustment knob 147. The knob 147 is mounted at the end of a wormscrew 148 that is eccentric relative to the axis 118. The screw 148 is supported at one end by the ring 145 which is provided with a recess for this purpose, and it is fixed at its other end to a rod 149 passing through the plug 141, and it forms the shaft of the adjustment knob 147. The source 146 is fixed to a source carrier 150 whose shape on one side matches the inside wall of the tubular body 140 so as to be slidable inside the tubular body, and which is provided with teeth adapted to engage with the wormscrew 148. It is thus possible to move the source 146 in translation inside the tubular body 140, in particular to bring it closer to the Venturi section, by turning the adjustment knob 147.

This maneuver can serve to compensate for the source's drop in activity over time, in an embodiment where the source is gadolinium 153. Another advantage of this arrangement, regardless of the type of source used, is that it makes it possible to adapt the photon flux emitted by the source 146 to the nature, and thus to the attenuation characteristics, of the multiphase fluid flowing through the Venturi section.

It should be observed that any displacement of the source 146 requires a new measurement of the “empty” count rate W_(hi,0) and W_(lo,0) since the photon flux reaching the detector is modified by displacing the source.

Like the source block 120, the detector block 121 includes a generally tubular body 160 closed at one end by a plug 161 and including at its other end, for assembly to the Venturi section, a portion 162 of smaller outside diameter that is designed to be screwed into the above-mentioned housing 131. A bore 164 passes through portion 162 and has two portions of different diameters so as to form a shoulder against which there bears on one side a ring 167 of a material of low photon absorption 165 at the energies emitted by source 146, the ring 167 being in abutment on the other side against the shoulder of housing 131.

A box 175 of cylindrical outside shape is located in the tubular body 160, said box being axially slidable relative to the body 160, a helical spring being mounted between box 175 and a recess formed in plug 161. The box contains a detector unit 177, 178 comprising, as mentioned above, a scintillator crystal, e.g. of sodium iodide, together with a photomultiplier. The above-mentioned circuit for processing pulses (not shown) is integrated in this detector unit and is thus placed inside the box 175. Conductors, generally referred to as numeral 179, connect the detector unit to an external power supply and the detector unit 159 to the data processing unit 30.

Description with reference to FIG. 4

The section view of FIG. 4 shows an advantageous way of using a device of the kind shown in FIG. 2. The principle consists in connecting a section of pipe which is adapted to taking measurements as described above to the production installation situated in the vicinity of a well head but does not include any measurement means proper (sensors and associated processor means). Such measurement section is permanently mounted on the main pipe connected to the well head, and valves are provided in the main pipe between itself and the measurement section so that when a measurement is to be performed, the well effluent can be diverted through the measurement section. Measurement means are assembled to the measurement section, thereby making up the device as shown in FIG. 2, only when a measurement is to be performed. This form of implementation, that can be referred to as “semi-periodic”, makes it possible to perform periodic monitoring of production from a well under conditions that are advantageous for the user. Since the measurement section is already in place, the operations necessary to perform a measurement are restricted to assembling the sensors onto the measurement section and that can be done quickly and simply. In addition, the sensors which constitute the expensive and fragile portion of the device are not left exposed other than while performing monitoring operations. This is particularly advantageous for the gamma ray attenuation sensor which includes a radioisotope. Also, the sensors constitute equipment that is very lightweight (compared with the measurement section) and that does not require special transport means. Conversely, the measurement section is a piece of equipment that is robust and simple and that can be left permanently on a production site without great risk.

FIG. 4 is a diagram showing a measurement section of the type shown in FIG. 2, but while not performing measurement operations. Compared with FIGS. 2 and 3, corresponding elements are given the same reference numbers plus 100.

The measurement section is given overall reference 210. It is mounted in parallel with a production pipe connected to a well head in such a manner that the effluent can flow through the measurement section in the direction represented by arrow F when it is desired to take measurements. The pipes are provided for this purpose with valves (not shown). In appropriate manner, the measurement section is disposed vertically and the flow through the measurement section is upward, as indicated by arrow F.

The measurement section includes recesses for receiving the components of a gamma ray attenuation measuring device, said recesses passing through the wall of the measurement section at two diametrically opposite positions. A first recess, for the source, is formed in a housing 230 opening out into the outside wall of the measurement section 210, and of a smaller diameter housing 232 opening out in the inside wall, and similarly a second recess for the detector comprises a housing 231 opening out in the outside wall and a smaller diameter housing 233 opening out in the inside wall. The housings 230 to 233 are made in appropriate manner respectively like the housing 130 to 133 described in detail with reference to FIG. 3. Similarly, the housings 232 and 233 receive windows 234 and 235 made in appropriate manner like the elements 134 and 135 shown in FIG. 3 so there is no need to describe them again. It is merely recalled that each of the elements 134 and 135 is designed to have low gamma ray attenuation, and has sealing means associated therewith, and in a variant the two elements may be formed by separate portions of a single annular part provided with appropriate sealing means. The elements 234 and 235 are also protected by respective plugs 270 and 271 screwed into the housings 230 and 231. During a measurement operation, the plugs are unscrewed and the source block and the detector block elements described with reference to FIGS. 2 and 3 are screwed into the housings 230 and 231.

For measuring pressure in the flow, the measurement section shown includes pressure takeoff orifices 216 and 217 analogous to the orifices 116 and 117 of FIG. 2. A threaded housing 226 connects each housing to the outside of the measurement section. A connector 274 is screwed into the housing and is itself connected, as shown diagrammatically in FIG. 4, to a hydraulic line 275 provided with an end coupling 276 and including a valve 277 that is closed except when performing measurements. To perform a measurement, it thus suffices to connect the coupling 276 to the pressure sensor and to open the valve 277.

The principles explained above are also applicable to the case mentioned above in which the measurement section is used for gamma ray attenuation measurements only. Under such circumstances, there is no need to provide a Venturi in the measurement section, nor is there any need to provide pressure takeoff orifices for measuring the pressure drop due to the Venturi. Under such circumstances, the measurement section may be of constant diameter and need only comprise the “windows” presenting low gamma ray attenuation and the elements associated therewith, as described above. 

We claim:
 1. A flow section adapted for measurements of oil well effluent at a well head, the section comprising: a) a measurement pipe connected in parallel with an effluent pipe connected to the well head, and having first and second opposed housings formed therein which open onto internal and external walls of the measurement pipe and are respectively adapted to receive removable gamma ray source means and gamma ray detector means, together defining gamma ray attenuation measuring means; b) elements of material having low gamma ray attenuation fixed in openings of the housings formed on the internal wall of the measurement pipe, and c) protection members removably disposed in openings in the housings connecting to the outside so as to close off the housings from the outside, the protection members being removable from the housings so as to allow removable gamma ray attenuation measuring means to be located in the housings adjacent to the elements.
 2. A section according to claim 1, in which sealing means are provided in association with said elements.
 3. A section according to claim 1, in which the measurement pipe includes a Venturi, the wall of the measurement pipe including at least two orifices opening out into the inside thereof, at least one of which is located at the throat of the Venturi and enabling the pressure drop that results from the Venturi to be measured, ducts being respectively connected to said orifices, said ducts being provided with closure means and being suitable for being connected to pressure measuring means.
 4. An oil well effluent measuring system, including a flow section according to claim 3, means for measuring gamma ray attenuation, including a source block and a detector block, each provided with means for fixing to the measurement pipe and organized to co-operate with said housings after the protection members have been removed, and pressure measuring means suitable for being connected to said orifices.
 5. A measurement system for oil well effluent, including a flow section according to claim 1 and gamma ray attenuation measuring means including a source block and a detector block, each provided with means for fixing to the pipe that are organized to co-operate with said housings after the protective members have been removed.
 6. A method of measuring oil effluent, comprising the steps of: a) permanently connecting a measurement pipe in parallel with an effluent pipe connected to the well head, the measurement pipe having first and second opposed housings formed therein which open onto internal and external walls of the measurement pipe and are adapted to receive removable gamma ray attenuation measuring means; elements of material having low gamma ray attenuation being fixed in openings of the housings formed on the internal wall of the measurement pipe, and protection members being removably disposed in openings in the housings connecting to the outside so as to close off the housings from the outside, the protection members being removable from the housings so as to allow removable gamma ray attenuation measuring means to be located in the housings adjacent to the elements, and b) removing protection members when it is desired to perform measurements; c) securing gamma ray source means in the first housing and gamma ray detector means in the second housing, said source and detector means each including fixing means organized to co-operate with the respective housings; d) measuring a gamma ray signal at the detector so as to determine a gamma ray attenuation signal; e) removing the source and detector means from the housings; and f) reattaching the protection members.
 7. Apparatus for measuring oil well effluent at a well head, comprising: a) an effluent pipe connected to the well head; b) a measurement pipe connected in parallel with the effluent pipe and having first and second opposed housings adapted to receive gamma ray attenuation measurement means formed therein, the housings having inner and outer openings defined in a wall of the measurement pipe, the inner openings being defined in an inner wall of the measurement pipe and the outer openings being defined in the outer wall of the pipe; c) windows formed from beryllium fixed in the inner openings; and d) protection members disposed in the outer openings so as to close off the housings from the outside, the protection members being removable from the housings so as to allow gamma ray attenuation measuring means to be at least temporarily located in the housings adjacent to the elements.
 8. Apparatus as claimed in claim 7, wherein sealing means are provided in association with the windows.
 9. Apparatus as claimed in claim 7, wherein the measurement pipe includes: (i) a Venturi; (ii) at least two measurement locations being provided in the wall of the measurement pipe by means of orifices defined therein which open into the inside thereof, at least one of the measurement locations being located at a throat of the Venturi for making a measurement of a pressure drop that resulting from flow through the Venturi; (iii) ducts suitable for being connected to pressure measuring means being connected to the measurement locations; and (iv) closure means being provided to close the ducts.
 10. Apparatus as claimed in claim 9, further comprising pressure measurement means connected to the ducts, the pressure measurement means measuring absolute pressure at each measurement location.
 11. Apparatus as claimed in claim 9, further comprising pressure measurement means connected to the ducts, the pressure measurement means measuring a differential pressure between the measurement location.
 12. Apparatus as claimed in claim 9, further comprising gamma ray attenuation measuring means including a source block and a detector block, each provided with means for fixing to the measurement pipe that co-operate with the housings after the protective members have been removed.
 13. Apparatus as claimed in claim 12, wherein the source block is fixed to the measurement pipe in a manner that allows its position relative to the Venturi to be adjusted.
 14. Apparatus as claimed in claim 7, further comprising gamma ray attenuation measuring means including a source block and a detector block, each provided with means for fixing to the measurement pipe that co-operate with the housings after the protective members have been removed.
 15. Apparatus as claimed in claim 14, wherein the source block emits gamma rays of at least two energy levels.
 16. Apparatus as claimed in claim 15, wherein the two energy levels comprise a higher energy level that is sensitive to effluent density and a lower level that is sensitive to effluent composition.
 17. Apparatus as claimed in claim 7, further comprising a valve for diverting substantially all flowing effluent from the effluent pipe into the measurement pipe when measurements are to be made.
 18. Apparatus as claimed in claim 7, wherein the beryllium windows are coated with boron hydride.
 19. Apparatus as claimed in claim 7, wherein the windows are formed by distinct parts of an annularly shaped part located in the pipe.
 20. A method of measuring oil effluent at a well head having an effluent pipe, the method comprising (a) permanently installing a measurement pipe in parallel with the effluent pipe, the measurement pipe including: (i) a Venturi; (ii) opposed housings defined in a wall of the measurement pipe; (iii) elements of low gamma ray attenuation material fixed in openings formed in housings defined in an inside wall of the measurement pipe level with a throat of the Venturi; (iv) protection members disposed in the housings connecting to the outside of the measurement pipe; (v) at least two measurement locations comprising orifices defined in the inner wall of the measurement pipe; (vi) ducts connected to the measurement locations; and (vii) closure means for the ducts; (b) removing the protection members; (c) securing the gamma ray attenuation measuring means in the housings; (d) removing the closure means; (e) connecting pressure measuring means to the ducts; (f) measuring the gamma ray attenuation using the attenuation measuring means; and (g) measuring a pressure drop in the measurement pipe with the pressure measurement means.
 21. A method as claimed in claim 20, further comprising: (h) removing the gamma ray attenuation measuring means after measuring the gamma ray attentuation; (i) reattaching the protection members; (j) removing the pressure measuring means after measuring the pressure drop in the measurement pipe; and (k) reattaching the closure means. 